Barely do any of them? He's physical and he's big, they are things that he is, not things he can do. In the first half of the season, we played with inexperienced CBs in an unfamiliar system. It meant that teams would press high, especially away from home, which would lead to going back to De Gea a lot. Having Fellaini on the pitch helps a lot because he's able to bring the ball down and allow us to keep possession in the opponents half. We are also shit on corners, so having Fellaini there helps defensively. Since we've got defenders back he's not played, he hasn't started a league game since Liverpool 14th December.
With regards to Rooney, bar the odd sloppy pass he does well in midfield. Switching the play allows us to attack with space in the final third and use the width effectively, something that's hard to create otherwise in wingback formations. He's a big goal threat from midfield obviously, but also teams will plan to nullify Rooney which is likely to present opportunities for our other talented players. I don't think opponents would focus on Herrera the same way. He's been better than Di Maria anyway for example, who loses the ball an incredible amount of times from "Hollywood" decisions to shoot or pass, plus he's better defensively. Of course though, Di Maria is our new £60million signing so we couldn't suggest dropping him. To which people will then inform me about all Di Maria's qualities and I'll agree because I'm a big fan of his, but of course if I talk about Rooneys qualities it'd get downplayed.
Herrera is our best midfielder I'd say in terms of keeping the ball and making correct decisions, wonderfully tidy player. In our current system though, we need players that are going to making things happen spontaneously because, as we can see, we never look like creating a lot. Herrera can pick a pass and can finish, but is he more likely to win a game than Rooney or Di Maria? I wouldn't say so.